BCD Counter
BCD stands for Binary Coded Decimal. A BCD counter has four outputs. By convention A is the least significant bit, or LSB. The easiest way to understand what a BCD counter does is to follow the counting sequence in truth table form. A BCD counter or decade counter can be constructed from a straight binary counter by terminating the "ripple-through" counting when the count reaches decimal 9 (binary 1001). Since the next toggle would set the two most significant bits, a NAND gate tied from those two outputs to the asynchronous clear line will start the count over after 9.
BCD counter symbol
BCD counter truth table
| CLK | Q3 | Q2 | Q1 | Q0 |
|---|---|---|---|---|
| 0 | 0 | 0 | 0 | 0 |
| 1 | 0 | 0 | 0 | 1 |
| 2 | 0 | 0 | 1 | 0 |
| 3 | 0 | 0 | 1 | 1 |
| 4 | 0 | 1 | 0 | 0 |
| 5 | 0 | 1 | 0 | 1 |
| 6 | 0 | 1 | 1 | 0 |
| 7 | 0 | 1 | 1 | 1 |
| 8 | 1 | 0 | 0 | 0 |
| 9 | 1 | 0 | 0 | 1 |